Angelman syndrome is a genetic disorder usually caused by a deletion of material on the proximal long arm of chromosome 15. Behavioral problems among children with Angelman syndrome include severe difficulties initiating and maintaining sleep. Physiological doses of melatonin, the main hormone of the pineal gland, have been shown to induce sleepiness in healthy young humans, to decrease latency to sleep onset and latency to stage 2 sleep, as well as to decrease latency to sleep during night awakenings. This study will examine the endogenous melatonin levels, temperature and activity patterns in children with Angelman syndrome, and the acute behavioral, temperature and hormonal changes that occur when these children are exposed to physiological doses of the hormone an hour before their bed time.